1. Field of the Invention
The present invention relates to a light source unit including a light source for performing thermally-assisted magnetic recording, and to a magnetic recording head provided with the light source unit. Further, the present invention relates to a method for manufacturing the light source unit.
2. Description of the Related Art
As the recording densities of magnetic recording apparatuses become higher, as represented by magnetic disk apparatuses, further improvement has been required in the performance of thin-film magnetic heads and magnetic recording media. As the thin-film magnetic heads, a composite-type thin-film magnetic head is widely used, which has a stacked structure of a magnetoresistive (MR) element for reading data and an electromagnetic transducer for writing data.
Whereas, the magnetic recording medium is generally a kind of discontinuous body of magnetic grains gathered together, and each of the magnetic grains has a single magnetic domain structure. Here, one record bit consists of a plurality of the magnetic grains. Therefore, in order to improve the recording density, it is necessary to decrease the size of the magnetic grains and reduce irregularity in the boundary of the record bit. However, the decrease in size of the magnetic grains raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume.
As a measure against the thermal stability problem, it may be possible to increase the magnetic anisotropy energy KU of the magnetic grains. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. Whereas, the intensity of write field generated from the thin-film magnetic head is limited almost by the amount of saturation magnetic flux density of the soft-magnetic material of which the magnetic core of the head is formed. As a result, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field of the medium exceeds the write field limit.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large energy KU is used so as to stabilize the magnetization, then anisotropic magnetic field of a portion of the medium, where data is to be written, is reduced by heating the portion; just after that, writing is performed by applying write field to the heated portion.
In this thermally-assisted magnetic recording technique, there has been generally used a method in which a magnetic recording medium is irradiated and thus heated with a light such as near-field light. In this case, it is significantly important to stably supply a light with a sufficiently high intensity at a desired position on the magnetic recording medium. However, from the beginning, more significant problem to be solved exists in where and how a light source with a sufficiently high output of light should be disposed inside a head.
As for the setting of the light source, for example, U.S. Pat. No. 7,538,978 B2 discloses a configuration in which a laser unit including a laser diode is mounted on the back surface of a slider, and US Patent Publication No. 2008/0056073 A1 discloses a configuration in which a structure of a laser diode element with a monolithically integrated reflection mirror is mounted on the back surface of a slider. Further, US Patent Publication No. 2005/0213436 A1 discloses a structure of slider that is formed together with a semiconductor laser, and Robert E. Rottmayer et al. “Heat-Assisted Magnetic Recording” IEEE TRANSACTIONS ON MAGNETICS, Vol. 42, No. 10, p. 2417-2421 (2006) discloses a configuration in which a diffraction grating is irradiated with a light generated from a laser unit provided within a drive apparatus.
As described above, various types of the setting of the light source are suggested. However, the present inventors propose a thermally-assisted magnetic recording head with a “composite slider structure” which is constituted by joining a light source unit provided with a light source to the end surface (back surface) of a slider provided with a write head element, the end surface being opposite to the opposed-to-medium surface of the slider. The “composite slider structure” is disclosed in, for example, US Patent Publication No. 2008/043360 A1 and US Patent Publication No. 2009/052078 A1. The advantages of the thermally-assisted magnetic recording head with the “composite slider structure” are as follows:
a) The head has an affinity with the conventional manufacturing method of thin-film magnetic heads because the opposed-to-medium surface and the element-integration surface are perpendicular to each other in the slider.
b) The light source can avoid suffering mechanical shock directly during operation because the light source is provided far from the opposed-to-medium surface.
c) The light source such as a laser diode and the head elements can be evaluated independently of each other; thus the degradation of manufacturing yield for obtaining the whole head can be avoided; whereas, in the case that all the light source and head elements are provided within the slider, the manufacturing yield rate for obtaining the whole head is likely to decrease significantly due to the multiplication of the process yield for the light-source and the process yield for the head elements.
d) The head can be manufactured with reduced man-hour and at low cost, because of no need to provide the head with optical components such as a lens or prism which are required to have much high accuracy, or with optical elements having a special structure for connecting optical fibers or the like.
There are two factors in improving the performance of a head having such a “composite slider structure”. One is reduction of tact time required for production of the head, in particular, production of a light source unit, and reduction of the cost of the production process.
In practice, a light source unit is fabricated by joining a light source such as a laser diode onto a unit substrate. A head is fabricated by coupling the light source unit to a slider. It is known that the distance between the light-emission center located in the light-emitting surface of the light source that emits light and a light-receiving end surface of an optical system such as a waveguide located on the back surface of the slider needs to be controlled to 10 μm (micrometers) or less in the production processes in order to provide a sufficiently high light use efficiency. Therefore the positional accuracy in joining the light source onto the unit substrate in the first place is critical. Furthermore, in order to avoid damage to the light source, the light-emitting surface of the light source should not protrude from the light source unit, of course. The production process of the light source unit that meets these requirements generally requires a considerably high mounting accuracy, and thus a very sophisticated image recognition technology and a highly accurate positioning mechanism. Consequently, a considerably expensive production facility is required for the production, and the tact time required for the production also tends to increase. Accordingly, per-head-chip cost can increase.
The other factor in improving the performance of the “composite slider structure” head is reduction of the weight of the light source unit. The light source unit is mounted on the slider and is mounted on a suspension as the head. To maintain and enhance the flying performance and impact resistance of the head in operation, it is important to sufficiently reduce the weight of the light source unit. US Patent Publication No. 2008/043360 A1 and US Patent Publication No. 2009/052078 A1 described above disclose the source-installation surface of the unit substrate on which terminal electrodes for the light source are provided. The source-installation surface needs the provision of at least an area where the light source is to be bonded and an area to be occupied by the terminal electrodes for the light source, which limits the reduction of the volume of the unit substrate to reduce the weight. Therefore, there is a need to reduce the weight of the light source unit while ensuring power supply to the light source.